Silver halide photography and silver halide diffusion transfer process are conventionally known as methods of high-sensitivity photography. Details of the former method are described in J. H. James, "The Theory of the Photographic Process", Macmillan, New York 1966, and those of the latter method are described in A. Rott and E. Weyde, "Photographic Silver Halide Diffusion Processes", The Focal Press, London and New York, 1972.
A photographic material having fairly high sensitivity is sold by Minnesota Mining & Manufacturing Company under the trade name "Dry-Silver" and details of this product are given in U.S. Pat. Nos. 3,152,903, 3,152,904 and 3,457,075.
These photographic techniques are characterized by high-sensitivity and rapid or dry processing to produce a high-quality image in their own way. But most of them require the use of much silver since silver halide is used as a photosensitive material and metallic silver is used as an image-forming material (or as an intermediate medium for dye-image formation in case of silver halide salt color photography). On the other hand, only part of the spent silver (i.e. silver used in image formation and lost in the processing solutions) is recovered for further use, and this is the primary reason why silver halide photography is expensive. Depletion of silver resources and fluctuations in silver price are two more reasons for making the silver salt photography unsuitable for use in today's energy-sensitive industry. Therefore, the development of a photographic process using a limited amount of silver or eliminating its use entirely is desired.
While many nonsilver photographic processes have been reported, most of them are of lower sensitivity than silver halide photography, and most of the nonsilver photographic materials are not capable of forming an image of continuous tone. Among the nonsilver photographic materials, those which use the grains of copper (I) halide crystal have a relatively high sensitivity and produce an image of continuous tone, and photographic methods that use these photosensitive materials are described in Research Disclosure Nos. 15166 and 15252. According to these references, the crystal of copper (I) halide is sensitive to ultraviolet rays and a photosensitive material prepared by coating a support with a dispersion of the grains of the crystal in a binder solution can produce a colored image of continuous tone by irradiation with UV rays and physical development. Harry T. Spencer and Jacqueline E. Hill studied a method of developing this photosensitive material and details of their study are given in Research Disclosure No. 15166 (1976). First, the crystal of copper (I) halide is dispersed in a solution of a binder such as cellulose acetate butyrate which is soluble in an organic solvent (e.g. acetone or acetonitrile) and the resulting dispersion is spread on a support and dried to prepare a copper (I) halide photosensitive material. It is exposed in a wet state as it is immersed in a polar solvent such as water, alcohols (e.g. methanol and ethanol) and glycols (e.g. ethylene glycol and diethylene glycol) or an alkaline developer wherein these solvents are mixed with alkylamines such as ethylenediamine, diethylene triamine and triethylenetetramine. Then, a disproportionating reaction takes place according to the following scheme (1) and metallic copper and a Cu.sup.2+ ion are formed: 2Cu.sup.+ .fwdarw.Cu.sup..degree. +Cu.sup.2+ (1). When the photosensitive material having metallic copper is developed with a solution containing alkylamine (with triethylenetetramine being particularly preferred), the alkylamine forms a complex with the Cu.sup.2+ ion. Since the metallic copper acts as a catalyst in disproportionation, the reaction proceeds rapidly in an area where metallic copper has been produced by exposure. This way, the photosensitive material is developed to provide a visible image.
The inventors of the present invention prepared an emulsion comprising fine grains of a copper (1) halide-containing silver halide crystal by mixing an aqueous solution containing Cu.sup.2+ ions and another aqueous solution containing a halogen ion and a reducing agent to reduce the Cu.sup.2+ ions in an aqueous gelatin solution in the presence of Ag.sup.+ ion. As stated in Japanese Patent Application No. 24669/81, a photosensitive material having this emulsion applied onto a support has sensitivity in a dry state. The inventors exposed this sensitive materil and tried to develop it with a mixture of 4-N-methylaminophenol hemisulfate or 1-phenyl-3-pyrazolidone and hydroquinone in a pH range of from 6 to 14, but no visible image could be formed at any of the pHs in that range. A visible image could be obtained when the photosensitive material was developed in an alkaline aqueous solution of alkylamine such as ethylenediamine or triethyleneamine by making use of the disproportionating reaction described above. However, development in an aqueous alkaline aolution of alkylamine of a photosensitive material prepared by coating a support with a copper (I) halide emulsion or the same emulsion containing silver halide has the following disadvantages: (1) the colored image formed by development has high fog density (color density of the unexposed area), (2) the image has low maximum color density, and (3) the image has low contrast.
Japanese Patent Application (OPI) No. 96531/77 (the symbol OPI as used herein means an unexamined published Japanese patent application) discloses a technique for producing a black nonsilver image from a copper compound. According to this technique, a photosensitive material comprising a support coated successively with a layer that contains a salt of monovalent or divalent copper or its complex and a hydrophilic binder and an emulsion layer containing a small amount of silver halide is exposed and developed to form an imagewise pattern of metallic silver (developed silver), and a small amount of this metallic silver is used as a direct catalyst for chemical development of the photosensitive material with a developer containing a reducing agent such as paraformaldehyde, formalin, amineboranes, sodium borohydride, L-ascorbic acid, pyrazolidones, aminophenols and polyhydroxybenzenes. But when the present inventors subjected this photosensitive material to imagewise exposure in a dry state or a wet state as it was immersed in a developer, and subsequently developed it with a solution containing L-ascorbic acid and diethanolamine or dimethylamineborane and triethanolamine, the following disadvantages resulted: (I) the colored image produced by the development had high fog density and/or the image had a low maximum color density, and (2) the image had low contrast.
The present inventors then made studies on various developers suitable for the copper (I) halide photosensitive material, and found that by developing it with an alkaline aqueous solution of an amino acid typified by L-glutamine or an amino acid derivative, a colored image having less fog than that obtained by development with an aqueous alkylamine solution (e.g. aqueous triethylenetetramine solution) or a solution containing dimethylamineborane and triethanolamine could be obtained, but the maximum color density and image contrast were not satisfactorily high.
The inventors therefore continued their search for improved developers by studying the mechanism of development of the copper (I) halide photosensitive material, as well as the defects of the conventional developers. As a result, they found that when a photosensitive copper (I) halide material containing no silver halide was immersed in a weakly alkaline aqueous solution of 4-N-methylaminophenol sulfate, then exposed in a wet state and developed with said solution, a photographic image that had low maximum color density and low contrast but which had no fog was produced. When the same photosensitive material was likewise processed with a weakly alkaline aqueous solution of L-ascorbic acid, a similar photographic image (low maximum density and contrast but having no fog) was obtained. It was found that by developing the copper (1) halide photosensitive material with the developer comprising an alkaline aqueous solution of an amino acid typified by L-glutamine or an amino acid derivative or an alkaline aqueous solution of an amine or an amine derivative, a colored image having less fog (e.g. color density at the unexposed area) but having a high maximum color density and a relatively high image contrast could be obtained, but the meximum color density and image contrast were not satisfactorily high. However, when the same processing was conducted with an alkaline aqueous solution containing both 4-N-methylaminophenol sulfate and L-ascorbic acid, or both L-ascorbic acid and .alpha.-amino acid, or both 4-N-methylaminophenol sulfate and .alpha.-amino acid, the developer had superadditivity and produced a photographic image of good quality that had a high maximum color density, a very low fog density, high contrast and a neutral tone. This is very surprising since no other developer composition has ever been known to exhibit such superadditivity in the development of the photosensitive copper (I) halide material.